Aqueous curable coating compositions which comprise polymers having hydrolysable silane groups are known. These compositions can form crosslinked coatings on removal of the water, either by subsequent reaction between the hydrolysable silane groups or alternatively by reaction of the silane groups with hydroxyl groups.
European Patent Application No. EP-A-280994 discloses electrodepositable coating compositions which comprise a polymer having ionic groups and hydroxyl groups and a crosslinking agent having at least two alkoxy silane groups. The polymer and crosslinking agent are dissolved or dispersed together in water.
A problem with compositions which rely for curing on reaction between silane groups and hydroxyl groups is that the resulting Si--O--C bond is polar and is unstable to moisture which gives rise to poor water resistance in the cured coating.
European Patent Application No. EP-A-276469 discloses an electrodepositable coating composition which comprises an aqueous dispersion or solution of a polymer having both ionic groups and alkoxy or hydroxy silane groups.
Published British Patent Application GB-A-2 215 339 discloses aqueous dispersions of a vinyl polymer which has a low level of silane functionality. Similar compositions are disclosed in British Patent GB-B-1407872.
British Patent GB-A-1,407,827 discloses aqueous dispersions of a vinyl polymer which has hydrolysable silane groups by virtue of the inclusion of a silane functional monomer during the process of making the polymer.
A problem with the use of polymers which are soluble in water and which contain hydrolysable silane groups is that these can only be made at relatively dilute solutions if they are to have a low viscosity. More concentrated solutions have an unacceptably high viscosity and are prone to premature intermolecular crosslinking and gellation.
A problem with dispersions of polymers having siloxane groups is that only low levels of silane can be incorporated into the polymer. The incorporation of higher levels of silane leads to ongoing intra-molecular crosslinking between silane groups within the dispersed particles. This results in very highly crosslinked particles which, on their own, are poor film-formers particularly at or near ambient temperature. The use of low levels of hydrolysable silane groups in the polymer leads to a low level of crosslinking in films formed from these compositions. This low level of crosslinking leads to poor final film properties such as poor water resistance and poor solvent resistance.